It’s somewhat moot anyway since I doubt if there are any approaches which haven’t long since had a GPS overlay.

No it is still the case if the aid is used for final approach lateral guidance. Most US runways have actual GPS approaches (or an overlay which is just a GPS approach that follows a conventional approach track) so as you say it doesn’t matter much there.

In the UK this is a significant improvement due to the prevalence of NDBs in missed approach segments of for example ILS or GPS approaches. But it isn’t a magic bullet and there remains the need to get more GPS approaches in the system.

Isn’t take-off/rotation distance a quadratic function of speed, like landing distance (i.e. double the touch-down speed and we roughly quadruple the braking distance)

Sure, but they’re not talking about aircraft performance but rather about what visibility/RVR the pilot needs for a safe take off. That would depend on the time it takes for the aircraft to travel a length of runway and that in turn is directly proportional to takeoff speed.

OK, so they are ignoring the time/distance required to accelerate to Vx or Vy? But to me that’s all that matters.

Perhaps I’m missing something obvious, but I still don’t see where that (arbitrary?) minimum RVR of 150 m came from, unless it’s just regulation for the sake of regulating.

We don’t often get bad weather in Scotland but when I take off from home with a bit of mist or hill fog, I just need to see far enough to back-track sedately to the top of the runway – i.e. 20-40 ft so I can see the runway edge cones. While back-tracking I can clear any dozy sheep or deer out of the way and check that there are no fallen trees or new holes dug by wild boar etc.

Then aim the HSI at 210 degrees, give it full gas, ask the nice lady in the audio panel to “say fuel flow”, lift the tail after 2-3 seconds and fly after another 3 – give or take a few seconds for wind, weight and DA. This is in a typical cheap touring taildragger, but my C150, C172, and C175 cheap touring nosedraggers all performed much the same.

Is it unreasonable to say that if we can’t hold an average HSI heading within +/- 5 degrees for five or ten seconds, we probably shouldn’t be flying in IMC?

What would be wrong with allowing the PIC to determine what minimum RVR she is comfortable with, taking account of all relevant conditions?

OK, so they are ignoring the time/distance required to accelerate to Vx or Vy? But to me that’s all that matters

What matters it the ability to maintain directional control.

Jacko wrote:

I still don’t see where that (arbitrary?) minimum RVR of 150 m came from, unless it’s just regulation for the sake of regulating.

Probably. I doubt they made any scientific evaluation how far you have to be able to see to maintain directional control.

Jacko wrote:

What would be wrong with allowing the PIC to determine what minimum RVR she is comfortable with, taking account of all relevant conditions?

The overwhelming number of fatalities caused by pilots trying to depart with lower visibility, and in particular the carnage of innocent bystanders standing right at the runway edge in fog, of course! [where is the sarcasm emoji?]

At least now the limit is now less than 400m, which is longer than the take off distance for many light aircraft…

I could not possibly comment on whether I have ever seen that, but anybody who has seen a departure in say 200m will tell you it is quite spooky. The scene in front of you is the same as you accelerate. Just see what looks the same piece of tarmac. All you see change is the airspeed rising. Then you pull up at Vr and immediately lose all visual references.

No it is still the case if the aid is used for final approach lateral guidance. Most US runways have actual GPS approaches (or an overlay which is just a GPS approach that follows a conventional approach track) so as you say it doesn’t matter much there.

Unfortunately, it doesn’t seem there is much to do about this. I checked in PANS-OPS and for NDB and VOR approaches, the narrowest part of the protected area (at the beacon) has a width of 2.5 and 2.0 NM respectively. For an LNAV approaches, the protected area has a width of 2.9 NM. So even of the directional accuracy is totally superior for LNAV — particularly compared to NDBs — there could be a significant obstacle which is not accounted for if you make an LNAV approach that just follows the approach track of a conventional NDB och VOR approach.

The take off minima of 400 metres with 150 metres under low visibility procedures and less still with the right qualifications and equipment plus approval from the competent authority has been part of the ICAO annexes for a long time. I think there has just been a relaxation in the 150m part.
I don’t think it was really written with pilots of light aircraft in mind but is more to do with airport operators being able to cover themselves for insurance purposes.
I.E. on many airport plates there is a 400 m minimum take off distance noted.Therefore there is either a declared RVR of 400m or the pilot has to declare that he can see 6 (I think or it might be 7 I need to look that up) runway markers when lined up on the threshold in France its called a VIBAL.
If the airport declares Low Visibility Procedures are in operation it is a warning to the airport emergency services to be on standby for take offs and landings and the pilot should be able to see a minimum of 2 runway markers when lined up at the threshold.
Below that needs special equipment, qualifications and permission. I don’t think they really care if Jacko takes off from his own airfield on 0/0 conditions, crashes his aircraft and kills himself:) (sorry Jacko), The 400m and 150m are calculated probably in the same way as obstacle clearance limits are calculated and probably agreed with insurance companies based on the risk of an aircraft in low visibility leaving the runway through loss of control and crashing into an Airbus full of passengers. Do you think the airport’s insurance company would pay out or the pilot’s insurance for that matter if you took off below the minimas and if there was no regulated minima it would be a good excuse for an insurance company to set its own minima, after the accident of course:)